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C H A P T E R

Metabolism and Energetics

17


Chapter 17 Learning Outcomes

• 17-1

• Define metabolism and energetics, and explain why cells need to synthesize new organic molecules.

• 17-2

• Describe the basic steps involved in glycolysis, the citric acid cycle, and the electron transport system, and summarize the energy yields of glycolysis and cellular respiration.

• 17-3

• Describe the pathways involved in lipid metabolism, and summarize mechanisms of lipid transport and distribution.

• 17-4

• Discuss protein metabolism and the use of proteins as an energy source.


Chapter 17 Learning Outcomes

• 17-5

• Discuss nucleic acid metabolism and the limited use of nucleic acids as an energy source.

• 17-6

• Explain what constitutes a balanced diet, and why such a diet is important.

• 17-7

• Define metabolic rate, describe the factors involved in determining an individual's BMR, and discuss the homeostatic mechanisms that maintain a constant body temperature.

• 17-8

• Describe the age-related changes in dietary requirements.


Nutrients and Energetics (17-1)

• Nutrients

• Essential substances needed for cells to generate

energy in the form of ATP

• ATP supports cell growth and division, contraction,

secretion, and other functions

• Energetics

• The study of flow of energy and how it changes from

one form to another


Metabolism (17-1)

• The sum total of all chemical reactions in the body

• Cellular metabolism provides energy needed for

homeostasis

• Catabolism

• Breakdown of organic molecules for synthesis of ATP

• Anabolism

• Synthesis of new organic molecules for storage


Figure 17-1 Cellular Metabolism.

INTERSTITIALFLUID

Plasma membrane

• Maintenance and repairs• Growth• Secretion• Stored nutrient reserves ANABOLISMCATABOLISM

NUTRIENT POOL

Organic Molecules

• Amino acids• Lipids• Simple sugars

Anaerobic catabolism in the cytosol releases small amounts of ATP that are significant only under unusual conditions.

Aerobic Metabolism(in mitochondria)

40%60%

CYTOPLASM

HEAT

Other ATP Expenses

• Locomotion• Contraction• Intracellular transport• Cytokinesis• Endocytosis• Exocytosis

Results of Anabolism


Four Reasons to Synthesize New Compounds (17-1)1. To perform structural maintenance and repairs

• Metabolic turnover is ongoing removal and

replacement of cell structures

2. To support growth

3. To produce secretions

4. To build nutrient reserves

• Glucose stored as glycogen, triglycerides in adipose

tissue


Nutrient Pool (17-1)

• Source for both catabolism and anabolism

• Anabolic activities require amino acids, some lipids, and

few carbohydrates

• Catabolic reactions break down carbohydrates first, then

lipids, and rarely amino acids

• Mitochondria use specific organic molecules to form

ATP, water, and carbon dioxide

• Utilizes the citric acid cycle and the electron transport

chain


Figure 17-2 Nutrient Use in Cellular Metabolism.

Structural, functional, and storage components

Triglycerides Glycogen Proteins

Amino acidsGlucoseFatty acidsNutrientpool

Small carbon chains

MITOCHONDRIA

CoenzymesCitric acid cycle

Electron transport system


Checkpoint (17-1)

1. Define energetics.

2. Define metabolism.

3. Compare catabolism and anabolism.


Carbohydrate Metabolism (17-2)

• Most cells use glucose to form ATP

• C6H12O6 + 6O2 6CO2 + 6H2O and 36 ATP

• Glucose + oxygen yields carbon dioxide + water and 36

molecules of ATP

• Anaerobic metabolism

• Does not require oxygen

• Aerobic metabolism or cellular respiration

• Requires oxygen, occurs in mitochondria


Glycolysis (17-2)

• Breakdown of glucose to two pyruvic acid molecules

• C6H12O6 2CH3—CO—COOH–

• Loses an H+, becomes pyruvate, enters mitochondria

• Glycolysis requires:

• Glucose and cytoplasmic enzymes

• ATP and ADP, NAD – a coenzyme that removes H+

• Invest 2 ATP, produce 4 ATP, for a net gain of 2 ATP


Figure 17-3 Glycolysis.

As soon as a glucose moleculeenters the cytosol, a phosphategroup is attached to the molecule.

A second phosphate group isattached. Together, steps 1 and 2cost the cell 2 ATP.

The six-carbon chain is splitinto two three-carbon molecules,each of which then follows therest of this pathway.

Another phosphate group isattached to each molecule, andNADH is generated from NAD.

The atoms in each three-carbonmolecule are rearranged and eachmolecule produces 2 ATP.

Glucose

Glucose-6-phosphatea

From mitochondria

To mitochondria

1,3-Bisphosphoglyceric acid

Pyruvate

To mitochondria

Steps in GlycolysisCYTOSOL

INTERSTITIALFLUID

ENERGY SUMMARY

Steps 1 & 2:Step 5:

–2 ATP+4 ATP

NET GAIN: +2 ATP


Mitochondrial Structure (17-2)

• Surrounded by two membranes

• Outer is permeable to pyruvate

• Inner uses carrier protein to transport pyruvate into

mitochondrial matrix

• Citric acid cycle occurs within mitochondria


The Citric Acid Cycle (17-2)

• Also called tricarboxylic acid (TCA) or Krebs cycle

• Pyruvate interacts with NAD and coenzyme A

(CoA)

• Product is acetyl-CoA

• Goes through citric acid cycle, transferring H+ to NAD

and FAD

• Forms ATP from GTP, NADH, and FADH2

• NADH and FADH2 transfer H+ to electron transport

system


Figure 17-4 The Citric Acid Cycle.

ELECTRONTRANSPORT

SYSTEM

An overview of the citric acid cycle, showing the distribu-tion of carbon, hydrogen, and oxygen atoms

Citric acid6-carbon

Coenzyme A

Pyruvate

Acetyl-CoA

4-carbon

5-carbonCITRICACID

CYCLE

Coenzyme A

4-carbon

(from GTP)


The Electron Transport System (17-2)

• ETS

• Embedded in inner mitochondrial membrane

• Provides 95 percent of all cellular energy

• A series of protein-pigment cytochromes

• Electrons from H+ enter ETS

• Travel along the ETS and release energy

• Energy drives H+ pumps creating gradient for H+

• Gradient energy converts ADP to ATP


Figure 17-5 The Electron Transport System (ETS) and ATP Formation.

Hydrogen atoms fromNADH and FADH2 aresplit into electrons andprotons. The protons (H+) are released and coenzyme Q passes the electrons to the electron transport system.

Innermembrane

Intermembranespace

Outermembrane

CYTOSOL

MITOCHONDRIALMATRIX

Flow of H+

Flow of electrons

KEY

ATPsynthase


Summary of Energy Yield from Carbohydrates (17-2)

• Glycolysis in cytoplasm yields 2 ATP and 2

pyruvate

• Citric acid cycle yields 2 ATP, one from each

pyruvate

• ETS yields 28 ATP

• Sum total from one molecule of glucose is 36 ATP

• All but two are produced in mitochondria


The Electron Transport System and Citric Acid Cycle (Aerobic):

Glycolysis (Anaerobic):

Energy Summary

net gain to cell fromcomplete catabolismof one glucose molecule

ELECTRONTRANSPORTSYSTEM

MITOCHONDRIA

Citric AcidCycle

(2 turns)

GLYCOLYSIS

Glucose(6-carbon) CYTOSOL

Pyruvate(3-carbon)

Acetyl CoA

Intermediateelectroncarriers

2

2

Figure 17-6 A Summary of the Energy Yield of Aerobic Metabolism.


Gluconeogenesis (17-2)

• The synthesis of "new" glucose from non-

carbohydrate molecules

• Needed because glycolysis is not a reversible reaction

• Pyruvate cannot be converted back to glucose

• Instead, cells use other 3-carbon molecules

• Lactate, glycerol, some amino acids

• Glucose from gluconeogenesis can undergo further

catabolic reactions to be stored as glycogen


CYTOSOL

Glycerol

Othercarbohydrates

3-carbon intermediates

Pyruvate

Acetyl-CoA

MITOCHONDRIA

Lactate

The reaction thatconverts pyruvate to acetyl-CoAcannot be reversed.

Some aminoacids

Glucose

Glycogen

GLUCONEOGENESIS

GLYCOLYSIS

Figure 17-7 Carbohydrate Metabolism.


Alternate Catabolic Pathways (17-2)

• Aerobic glucose metabolism is most efficient

• Without oxygen, cells will die

• Without glucose, cells find alternatives

• First, will switch to lipid-based ATP production

• Proteins catabolized only under starvation conditions

• Nucleic acids rarely catabolized for energy



Fatty acids Glycerol Glucose Amino acids

Pyruvate

Acetyl CoA

Citricacidcycle

MITOCHONDRIA

Figure 17-8 Alternate Catabolic Pathways.


Checkpoint (17-2)

4. What is the primary role of the citric acid cycle in

the production of ATP?

5. Hydrogen cyanide gas is a poison that produces

its lethal effect by binding to the last cytochrome

molecule in the electron transport system. What

effect would this have at the cellular level?

6. Define gluconeogenesis.


Lipid Catabolism (17-3)

• Also called lipolysis

• Hydrolysis reaction breaks triglycerides into glycerol and

three fatty acids

• One 18-carbon fatty acid can yield 144 ATP

• Glycerol is converted to pyruvate, enters citric acid cycle

• Fatty acids undergo beta-oxidation

• Occurs in mitochondria, generates NADH and FADH2

• Some fragments combine to form ketone bodies


Lipids and Energy Production (17-3)

• Lipids are stored in fat droplets

• Difficult for water-soluble enzymes to reach lipid

• Lipids can provide a lot of energy, but not quickly

• Skeletal muscle fibers access glucose for quick energy

• Skeletal muscle fibers access fatty acids during rest


Lipid Synthesis (17-3)

• Also called lipogenesis

• Glycerol is synthesized from a product of

glycolysis

• Other lipids, steroids, and fatty acids are derived

from acetyl-CoA

• Body cannot build every fatty acid it needs

• Essential fatty acids must be in diet


Lipid Transport and Distribution (17-3)

• Lipids not soluble in water, need transport

mechanisms

• Most circulated in plasma as lipoproteins

• Free fatty acids (FFA)

• Come from catabolized adipose tissue, available during

starvation periods

• Bind to albumin for transport


Lipoproteins (17-3)

• Contain triglycerides and cholesterol, covered in

phospholipids and proteins

• Chylomicrons in intestinal epithelium are largest

• Low-density lipoproteins (LDLs)

• "Bad cholesterol" deposited in arterial plaques

• High-density lipoproteins (HDLs)

• "Good cholesterol" transports cholesterol to liver


Lipoproteins and Lipid Transport and Distribution

The liver absorbs chylomicrons from the blood-stream and removes the triglycerides. It combines the cholesterol from the chylomicron with synthe-sized or recycled cholesterol, and alters the surface proteins. It then releases low-density lipoproteins (LDLs) into the circulation, which deliver cholesterol to peripheral tissues. Some of the cholesterol is used by the liver to synthesize bile salts; excess cholesterol is excreted in the bile.

The HDLs returnthe cholesterol to the liver where it is extracted, and packaged in new LDLs or excreted with bile salts in bile.

The LDLs released by the liver leave the bloodstream and enter capillaries.

Once in peripheral tissues, the LDLs are absorbed.

The cholesterol not used by the cells re-enters the bloodstream. There it gets absorbed by high density lipoproteins (HDLs).

The cell extracts the cholesterol and uses it in various ways.

Excess cholesterol isexcreted withthe bile salts

Lowcholesterol

Highcholesterol

Cholesterolextracted

Lysosomalbreakdown

Used in synthesisof membranes,hormones, other materials

Triglyceridesremoved

Chylomicrons

Cholesterolrelease

Figure 17-9 Lipoproteins and Lipid Transport.


Checkpoint (17-3)

7. Define lipolysis.

8. Define beta-oxidation.

9. Why are high-density lipoproteins (HDLs)

considered beneficial?


Protein Metabolism (17-4)

• Usually proteins are recycled inside cells

• Proteins are broken down, amino acids are used to

synthesize new proteins

• If carbohydrates and lipids are not available for

energy:

• Amino acids can undergo catabolism in the citric acid

cycle

• ATP benefits vary, but average that of carbohydrates


Amino Acid Catabolism (17-4)

• Removal of amino group requires vitamin B6

• Transamination moves amino group to other small carbon chain

• Forms "new" amino acid

• Deamination prepares amino acid for the citric acid cycle

• Removes amino group and forms ammonium ion (NH4+)

• Liver cells combine CO2 with NH4+ to produce urea

• Eliminated in urine



• Carbon chains, depending on structure, can:

• Be converted to pyruvate and go through

gluconeogenesis

• Or be converted to acetyl-CoA and go through the citric

acid cycle

• Or be converted to ketone bodies

• Acetone is one example, gives breath distinctive odor

• Ketone bodies can be used by other cells that convert them

back into acetyl-CoA

• High ketone body concentration is called ketosis



• An impractical source of quick energy because:

1. Proteins are more difficult to break apart than fats and

carbohydrates

2. Ammonium ions are toxic to cells

3. Structural proteins are essential for homeostasis

• Catabolism can alter cellular and systemic functions


Amino Acid and Protein Synthesis (17-4)

• Nonessential amino acids

• Can be synthesized in cells through amination or

transamination

• Essential amino acids

• Must be included in diet

• Include isoleucine, leucine, lysine, threonine,

tryptophan, phenylalanine, valine, methionine, arginine,

and histidine


Protein Deficiency Diseases (17-4)

• Inadequate consumption of essential amino acids

• Caused by malnutrition

• Inherited metabolic disorders

• For example, phenylketonuria (PKU)

• Cannot convert phenylalanine to tyrosine due to defective

enzyme

• Reaction essential to produce norepinephrine, epinephrine, and

melanin

• Infants' CNS development is inhibited


Checkpoint (17-4)

10. Define transamination.

11. Define deamination.

12. How would a diet deficient in vitamin B6 affect

protein metabolism?



Glucose

Fatty acids Glycerol Amino acids

Pyruvate

Acetyl-CoA

Citricacidcycle

Electrontransportsystem

Coenzymes

carry H

MITOCHONDRIA

Gluconeogenesis

Glycolysis

LIPIDS CARBOHYDRATES PROTEINS

KEY

Catabolic pathway

Anabolic pathway

Figure 17-10 A Summary of Catabolic and Anabolic Pathways.


RNA Catabolism (17-5)

• Broken into individual nucleotides

• Most are recycled

• Some broken down, but only sugar, cytosine, uracil can

go through the citric acid cycle to generate ATP

• Adenine and guanine are deaminated, excreted as uric

acid

• Hyperuricemia can cause crystals to form

• Causes gout


Nucleic Acid Synthesis (17-5)

• DNA synthesis occurs only in mitosis and meiosis

• Most cells synthesize RNA

• Transcribed by RNA polymerase

• Messenger RNA only when needed and is short-lived

• Ribosomal RNA and transfer RNA are longer lasting


Checkpoint (17-5)

13. Why do cells not use DNA as an energy source?

14. What are nitrogenous wastes?

15. Elevated levels of uric acid in the blood could

indicate an increased catabolic rate for

which type of macromolecule?


Adequate Nutrition (17-6)

• Nutrition

• The absorption of essential nutrients

• Balanced diet

• Contains all nutrients needed for homeostasis

• Prevents malnutrition


Food Groups and MyPlate (17-6)

• USDA food guide www.choosemyplate.gov

• Five basic food groups

1. Grains

2. Vegetables

3. Fruits

4. Dairy

5. Protein


Balanced Diets (17-6)

• Meet quantity and quality requirements

• Complete proteins

• Found in meat, eggs, dairy

• Have all essential amino acids and vitamin B12

• Incomplete proteins

• Found in grains, fruits, and vegetables

• Vegetarians require balance and fortified cereal or tofu


FruitsGrains

Vegetables

Protein

Dairy

ChooseMyPlate.gov

Figure 17-11 The MyPlate Food Guide.


Table 17-1 Basic Food Groups and Their Effects on Health


Minerals (17-6)

• Inorganic ions from dissociation of electrolytes

• Required in diet because:

1. Na+ and Cl– determine osmotic concentration of body

fluids

2. Various ions needed in maintaining membrane

potential, muscle contraction, buffers, etc.

3. Ions function as cofactors in enzymatic reactions


Table 17-2 Minerals and Mineral Reserves


Fat-Soluble Vitamins (17-6)

• Vitamins A, D, E, and K

• Absorbed from the digestive tract

• Can be stored easily in cells

• Avitaminosis (or vitamin deficiency disease)

• Rarely occurs due to lack of fat-soluble vitamins

• Hypervitaminosis

• Excess fat-soluble vitamins build up in storage


Water-Soluble Vitamins (17-6)

• Most are components of coenzymes

• Easily absorbed from the gut

• Excess readily excreted in urine

• Gut bacteria help prevent avitaminosis

• Hypervitaminosis

• Occurs rarely and usually when taking excessive

vitamin supplements


Table 17-3 The Fat-Soluble Vitamins


Table 17-4 The Water-Soluble Vitamins


Water (17-6)

• Basic daily requirement is 2.5 L/day

• May need more or less based on activity and body temp

• Most obtained through consumed food and water

• Some a product of ETS in mitochondria


Diet and Disease (17-6)

• Long-term issues result from unbalanced diet

• U.S. diets too high in sodium, lipids, and calories

• Results are epidemic-proportioned incidence of:

• Obesity

• Heart and vascular diseases

• Hypertension

• Diabetes


Checkpoint (17-6)

16. Identify the two types of vitamins.

17. What is the difference between foods described

as containing complete proteins and those

described as containing incomplete proteins?

18. How would a decrease in the amount of bile

salts in the bile affect the amount of vitamin A in

the body?


Units of Energy (17-7)

• Calorie – unit of heat energy

• Metabolic reactions release heat

• Calorie (cal) is the amount of energy required to raise

temp of 1 gram of water 1 degree Celsius

• Kilocalorie (kcal) or Calorie (Cal)

• More practical in human metabolic studies

• Amount of energy required to raise I kg of water 1oC


The Energy Content of Food (17-7)

• Calorimeter

• A test chamber where food, water, and oxygen are

ignited

• Food is completely burned to ash

• Water temp before and after test is compared

• Catabolism of:

• Lipids release 9.46 Cal/g

• Proteins and carbohydrates 4.32 and 4.18 Cal/g,

respectively


Energy Expenditure: Metabolic Rate (17-7)

• Measure of sum of all anabolic and catabolic

reactions in body

• Units of Cal/hour, Cal/day, or Cal/unit body weight/day

• Basal metabolic rate (BMR)

• Testing done under resting, fasting conditions

• Average BMR is 70 Cal/hour

• Variations due to age, sex, condition, weight, and

genetics


Resting Slowwalking

Speedwalking

Climbingstairs

Jogging Competitiveswimming

1000

800

600

400

200

0

Cal

ori

es p

er h

ou

r

Estimated calories expendedby a 70 kg (154 lb) individual

Figure 17-12 Caloric Expenditures for Various Activities.


Balance of Caloric Intake and Output (17-7)

• Weight gain occurs if energy intake exceeds

output

• Weight loss occurs if energy intake is lower than

output

• Best weight-control programs have both:

• Calorie counting

• Increase in daily exercise


Thermoregulation (17-7)

• Homeostatic process of maintaining body

temperature

• In spite of external environment

• Required to maintain enzymatic viability

• Body temp below 36oC or above 40oC causes

disorientation

• Above 42oC causes convulsions and permanent cell

damage


Mechanisms of Heat Transfer (17-7)

1. Radiation

• Accounts for more than half of body's heat loss

• Heat is lost as infrared radiation

2. Conduction

• Direct transfer due to physical contact with cold object

• Not very effective in gaining or losing heat


Mechanisms of Heat Transfer (17-7)

3. Convection

• Conductive heat loss to the air

• Heat rises away from skin, cool air replaces it

4. Evaporation

• Water changes from liquid to vapor, absorbing energy

• Sensible perspiration from sweat glands varies

• Insensible perspiration from lungs and skin is consistent


Radiation

Evaporation

Convection

Conduction

Figure 17-13 Mechanisms of Heat Transfer.


Mechanisms of Temperature Balance (17-7)

• Altering heat gain and heat loss activities

• Coordinated by heat-loss center and heat-gain center

• In hypothalamus

• Heat-loss uses parasympathetic pathways

• Heat-gain uses sympathetic pathways


Promoting Heat Loss (17-7)

• When body temp rises above set point:

1. Vasomotor center inhibited

• Peripheral blood vessels dilate

• Increased radiation and convection

2. Sweat glands stimulated

• Increased evaporation

3. Respiratory center stimulated

• Increased evaporation from lungs


Promoting Heat Gain (17-7)

• To prevent hypothermia, heat-loss center is

inhibited, heat-gain center is activated

• Stimulates vasomotor center

• Vasoconstriction limits radiation, convection, conduction

• Shivering thermogenesis

• Activates brief contractions of skeletal muscles

• Nonshivering thermogenesis is stimulated hormonally

• Thyroid hormones and epinephrine


Checkpoint (17-7)

19. Compare a pregnant woman's BMR (basal

metabolic rate) to her BMR when she is not

pregnant.

20. Under what conditions would evaporative

cooling of the body be ineffective?

21. What effect would vasoconstriction of peripheral

blood vessels have on body temperature on a

hot day?


Caloric Needs Decline with Age (17-8)

• Nutritionally required quality and balance of food

doesn't change with age

• Caloric requirement does decrease about 10

percent after age 50

• Need for calcium and vitamin D3 increases

• Need to support skeletal system

• Elderly on low income may lack protein and iron


Checkpoint (17-8)

22. Which changes with aging: nutritional

requirements or caloric requirements?